1. Field of the Invention
This invention is a circuit for correcting for poylgon scan errors in a raster input or output scanner, and more specifically is a high speed digital circuit clocked by a fixed frequency sample clock to produce a bit clock which has a variable frequency to effect scan correction.
An optical scanner may consist of a rotating light deflector which displaces a spot of light across a flat "scan line". "Scan deflectance" is a function which relates the position of the spot on the scan line to the angle of the deflector. Its angular derivative "scan revolvance", relates the change in position of the spot on the scan line to the angle of the deflector. A bit clock is used to regulate the flow of data into or out of the scanner system.
For the scanning system to function properly, there must be a well defined relationship between the bit clock frequency and the scan revolvance. For example, if a constant density of information on the scan line is required, then it may be appropriate to use a bit clock of constant frequency and a constant scan revolvance. Generally, the uncorrected scanner's scan deflectance is non-linear. Therefore, its scan revolvance is not constant. For instance, a typical cause of non-linearity is that the distance of the spot on the scan line to the center of rotation of the deflector increases as the spot moves away from the center of the scan line. Other non-linearities may be caused by the particular design of the scanner system. Since the bit clock used to regulate the flow of data into or out of the scanner system is generally constant during a scan, efforts have been made to make the scan revolvance constant also. Lenses may be employed to correct for the above-mentioned non-linearity by bending the spot toward the center of the scan line as the spot moves away from the center.
The speed at which the spot moves along the scan line is called "scan velocity". Ideally, scan velocity is proportional to scan revolvance. However, there are several factors which affect the constant of proportionality.
First, a scanning system may employ a series of deflectors, called facets, in a poligonal arrangement to increase the efficiency of the scanner system. Due to manufacturing anomalies, all the facets of the polygon are not necessarily at the same optical distance from the scan line. This means that each facet has its own constant of proportionality relating scan revolvance to scan velocity. This is called "polygon signature". Second, the deflector(s) may not turn at a constant rate. This is called "motor hunt". These system scanning errors may be minimized by either improving the polygon and drive systems, or by varying the clock rate to compensate for the errors.
2. Prior Art
In the inventor's prior patent applications, Facet-To-Facet Motor Hunt Correction and Polygon Signature Correction, an electronic method of correcting the bit clock rate was described. A sample clock having a fixed frequency slightly greater than N times the bit clock is first generated. From this stream of sample clocks, the exact number of evenly spaced pulses is removed so that, after being counted down by a factor of N, the resulting bit clock frequency is exactly correct to compensate for the motor hunt and signature errors for the current facet. The circuit shown uses a microprocessor to generate the number corresponding to the number of pulses between deleted sample clock pulses. This number is used to load a counter which is clocked by the sample clock. Every overflow is sent to a circuit which deletes one pulse from the stream of sample clocks. The modified sample clock bit stream is then counted down by N to create the system bit clock. A problem with this circuit is that there are too many possible frequencies to store in a table look-up, and a microprocessor is not fast enough to calculate corrections in real time.